The number of submarine cable systems that transmit and receive a light signal between a plurality of ground stations through a submarine cable has been increasing. In a submarine cable system, a light signal is transmitted and received not only between two stations, but also among three or more ground stations, by branching a transmission line and arranging a ROADM device and a third station. In this case, an optical fiber for transmitting and receiving a light signal is arranged in a submarine cable together with a power supply path for transmitting power.
In the above-mentioned submarine cable system, a repeater for amplifying light intensity of a light signal to be transmitted within the submarine cable is generally arranged between the stations. The repeater operates by power supplied from the ground station through the power supply path. Accordingly, for example, when supply of power to the repeater is interrupted due to an occurrence of an insulation failure or the like in the power supply path within the submarine cable, transmission and reception of a light signal between the stations are also interrupted. Therefore, even when a failure has occurred in the submarine cable or the like, there is an increasing demand for a submarine cable system capable of continuously using existing functions, to the maximum, by utilizing equipment in which no failure has occurred.
A technique for continuously using existing functions by utilizing equipment in which no failure has occurred when a failure has occurred in a submarine cable or the like is disclosed in, for example, PTL 1. In the technique in PTL 1, both-side power supply from an A-station and a B-station, which are trunk stations, is performed during a normal operation. For example, when a failure has occurred in a power supply line on the A-station side, the power supply line on the A-station side is grounded and a power supply line on the B-station side is connected to a power supply line on a C-station side, which is a branch station, thereby performing one-side power supply from the B-station and continuing transmission and reception of a light signal between the B-station and the C-station.